DEVELOPMENT OF AN ENVIRONMENTAL FLOW MANAGEMENT STRATEGY FOR THE THOMSON RIVER, VICTORIA, AUSTRALIA

The headwater section of the Thomson River has been nominated for list ing as a heritage river, mainly because it supports a population of la rge blackfish, a species which has high conservation value in Victoria . The upper river was dammed in 1983, creating an impoundment of 1.2 x 10(6) Ml capacity. It is anticipated that the dam will provide a reli able water supply to Melbourne for up to 20 years, but minimization of downstream releases will delay augmentation. For the period during wh ich the dam was filling, an interim environmental flow was applied. Ho wever, these flows were lower than what was considered at the time to be ideal for maintaining in-stream habitat in the long term. Despite p rofound modification of the river's hydrology, there is no evidence fo r serious environmental impacts. Macroinvertebrate populations have re covered from disturbance during the construction phase, and the divers ity of fish has not changed. However, there is concern that a lack of floods will result in contraction of the channel. This would probably mean a loss of available habitat area in the long term. Abstraction of water from the lowland section of the Thomson River began in 1957. Un favourably low flows have occurred since regulation, but wetland inund ation floods still occur with the same frequency. Although current man agement practices do allow unfavourable flow conditions to occur occas ionally, the regulated flow regime has not reduced the diversity of na tive fish present in the lower river. This is heartening, but given th e likelihood of future increases in the demand for water, long-term pr otection of in-stream fauna requires the application of an appropriate environmental flow regime. Habitat area-discharge curves derived from fish hydraulic preference data were used as the basis for devising a minimum flow recommendation. Five different methods of specifying mont hly flow regimes are compared. A method is presented that uses hydraul ic geometry relations from a neighbouring, hydrologically similar catc hment to specify the magnitude and duration of an artificial annual ch annel maintenance flood. Implementation of the suggested environmental flow regime will probably result in a flow deficit in the lower Thoms on River if current irrigation demands are to be met. To offset the de ficit, additional water can be released from the dam. The flow require d will not reduce the amount of habitat available in the upper Thomson River below the dam, but this strategy will bring forward the date of augmentation.